The present invention relates to a color cathode ray tube having a magnetism shield, and more particularly, to a color cathode ray tube having a magnetism shield, which is capable of protecting the inside of the color cathode ray tube from an environmental magnetic field.
Generally speaking, in a conventional color cathode ray tube, red, green and blue phosphors are rendered luminescent by a video signal in order to realize a color image. As shown in FIG. 1, such a conventional color cathode ray tube is made in the outer form of a bulb in which a panel 1 and funnel 9 are integrally formed. Inside a neck 10 placed at the rear of funnel 9 is provided an electron gun 11 for emitting red, green and blue electron beams 7. In panel 1, a mask frame 4 is supported by a plurality of support springs (not shown). On the panel side of mask frame 4, a shadow mask 3 is fixed to selectively pass the electron beams emitted from the electron gun. On the funnel side of mask frame 4, an inner shield 5 is secured to prevent the electron beams emitted from the electron gun from being distorted due to a terrestrial magnetic field or a leakage magnetic field. A phosphorous surface 2 coated with phosphors is formed on the inner surface of panel 1 so that the electron beams passing through shadow mask 3 collide with the phosphorous surface to form an image. The phosphorous surface 2 is divided into a pixel portion of red, green and blue phosphors for realizing red, green and blue color information, and a black matrix of graphite for shielding light between pixels.
In such a conventional color cathode ray tube, an environmental magnetic field caused due to the difference of a terrestrial magnetic field or due to sources of creating an abnormal magnetic field inside or outside a television or monitor, alters the path movement of the electron beams that extremely affecting color purity, convergence, raster displacement, and the like. For this reason, a shield mechanism made of ferromagnetic substance, such as shadow mask 3, mask frame 4 and inner shield 5, is incorporated in the form of a magnetic closed loop X in the conventional color cathode ray tube, as shown in FIG. 2.
FIG. 2 is intended to explain the magnetism shielding function of such a magnetic closed loop. According to this drawing, environmental magnetic field Y like the terrestrial magnetic field entering magnetic closed loop X is shielded by the magnetic closed loop X so that the inside of the magnetic closed loop X becomes a non-magnetic field area.
However, according to the environmental magnetic closed loop X of the conventional color cathode ray tube, only the area surrounding the shadow mask 3, mask frame 4 and inner shield 5, becomes the magnetic shield area not the overall inside area of the conventional color cathode ray tube. Until electron beams 7 deviate from the magnetic closed loop X and then reach phosphorous surface 2 from shadow mask 3, the path movement of the electron beams is distorted due to the environmental magnetic field, thus, deteriorating quality such as color purity, as described above. Especially, the magnetic closed loop of the conventional cathode ray tube cannot exhibit the shielding function against the environmental magnetic field coming externally from the phosphorous surface 2.
If the distance between the shadow mask and the phosphorous surface is small, no problems will exist. However, in a color cathode ray tube using a shadow mask, the distance of about 10 mm is required in the operation configuration so that the space between shadow mask 3 and phosphorous surface 2 is unable to escape from the effect of the outer environmental magnetic field.